MOX fuel

Mixed oxide, or MOX fuel, is a blend of oxides of enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors that predominate nuclear power generation.

transuranics and has been used in a few reactors to date, but is usually referred to specifically as thorium-plutonium rather than MOX to avoid confusion with uranium-plutonium MOX fuel.

One attraction of MOX fuel is that it is a way of disposing of surplus weapons-grade plutonium, which otherwise would have to be disposed as nuclear waste, and would remain a nuclear proliferation risk.

Overview

In every uranium-based nuclear reactor core there is both plutonium-238 (Pu-238) are formed similarly from U-235.

Normally, with the fuel being changed every three years or so, most of the Pu-239 is "burned" in the reactor. It behaves like U-235, with a slightly higher cross section for fission, and its fission releases a similar amount of energy. Typically about one percent of the mass number isotopes) in the mix decreases and even-mass number neutron-absorbing isotopes increase, requiring the total plutonium and/or enriched uranium percentage to be increased. Today in thermal reactors plutonium is only recycled once as MOX fuel, and spent MOX fuel, with a high proportion of minor actinides and even-mass plutonium isotopes, is stored as waste.

Re-licensing precedes the introduction of MOX fuel into existing nuclear reactors. Often only a third to half of the fuel load is switched to MOX. The use of MOX does change the operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it. More control rods are needed. For more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The Palo Verde Nuclear Generating Station near Phoenix, Arizona was designed for 100% MOX core compatibility but so far have always operated on fresh low enriched uranium. In theory the three Palo Verde reactors could use the MOX arising from seven conventionally fueled reactors each year and would no longer require fresh Uranium fuel.

According to the AECL, CANDU reactors could use 100% MOX cores without physical modification. Atomic Energy of Canada Limited (AECL), reported to the United States National Academy of Sciences committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3% plutonium.

Current applications

research reactor fuels except Japan, which wants no such weapons.

Thermal reactors

Over 30 thermal reactors in Europe (Belgium, Switzerland, Germany and France) are using MOX and a further 20 have been licensed to do so. Most reactors use it as about one third of their core, but some will accept up to 50% MOX assemblies. In France, EDF aims to have all its 900 MWe series of reactors running with at least one-third MOX. Japan aims to have one third of its reactors using MOX by 2010, and has approved construction of a new reactor with a complete fuel loading of MOX.

Fast reactors

Because the fission to capture-neutron cross-section with high energy or fast neutrons changes to favour plutonium and higher actinides as fuel. Depending on how the reactor is fueled it can either be used as a plutonium breeder or as a fast burner.

These fast reactors are better suited for the transmutation of other actinides than are thermal reactors. Because Thermal Reactors use slow or moderated neutrons the actinides which are not fissionable with thermal neutrons tend to absorb the neutrons instead of fissioning. This leads to build up of higher isotope actinides and lowers the number of thermal neutrons available to continue the chain reaction.

All plutonium isotopes are either fissile or fertile; in thermal reactors isotopic degradation limits the plutonium recycle potential. Along with Uranium about 1% of spent fuel is plutonium broken down as 40% Pu-239, and about 32% Pu-240, 18% Pu-241, 8% Pu-242 and 2% Pu-238 when the fuel is first removed from the reactor.

Fabrication

The first step is separating the plutonium from the remaining uranium (about 96% of the spent fuel) and the fission products with other wastes (together about 3%). This is undertaken at a nuclear reprocessing plant.

Dry mixing

MOX fuel can be made by grinding together uranium oxide (UO₂) and plutonium oxide (PuO₂) before the mixed oxide is pressed into pellets, but this process has the disadvantage of forming lots of radioactive dust. MOX fuel, consisting of 7% plutonium mixed with depleted uranium, is equivalent to uranium oxide fuel enriched to about 4.5% U-235, assuming that the plutonium has about 60- 65% Pu-239. If weapons-grade plutonium were used (>90% Pu-239), only about 5% plutonium would be needed in the mix.

Coprecipitation

A mixture of plutonium dioxide. The resulting powder can be converted using a base into green pellets using a press. The green pellet can then be sintered into mixed uranium and plutonium oxide pellet. While this second type of fuel is more homogenous on the microscopic scale (scanning electron microscope) it is possible to see plutonium rich areas and plutonium poor areas. It can be helpful to think of the solid as being like a salami (more than one solid material present in the pellet). In the following picture of MOX voids are seen in the plutonium rich phases, these are voids which formed during irradiation.

Americium content

Plutonium from reprocessed fuel is usually fabricated into MOX as soon as possible to avoid problems with the photons it emits are low in energy, so 1 mm of lead, or thick glass on a glovebox will give the operators a great deal of protection to their torsos. When working with large amounts of americium in a glovebox, the potential exists for a high dose of radiation to be delivered to the hands.

As a result old reactor-grade plutonium can be difficult to use in a MOX fuel plant, as the Pu-241 it contains decays with a short 14.1 year half-life into more radioactive PUREX or another aqueous reprocessing method.

Also, Pu-244 has an even longer halflife, but is unlikely to be formed by successive neutron capture because Pu-243 quickly decays with a halflife of 5 hours giving Am-243.)

Curium content

It is possible that both lead and water to protect the workers.

Also, the neutron irradiation of curium generates the higher used nuclear fuel; this has the potential to pollute the fuel cycle with strong neutron emitters. As a result, it is likely that curium will be excluded from most MOX fuels.

References

Technical Aspects of the Use of Weapons Plutonium as Reactor Fuel
Synergistic Nuclear Fuel Cycles of the Future
Nuclear Issues Briefing Paper 42
Burning Weapons Plutonium in CANDU Reactors
Program to turn plutonium bombs into fuel hits snags